The instant disclosure relates generally to satellites. In particular, the instant disclosure relates to apparatus and methods for maintaining a satellite in an artificial geosynchronous orbit, such as a geostationary orbit at an altitude other than the natural altitude of 35,786 km.
A “geosynchronous orbit” is an orbit around Earth, the orbital period of which is equal to a sidereal day (i.e., 23 hours, 56 minutes, 4 seconds), irrespective of the inclination of the orbit. A geosynchronous orbit need not be circular. The apparent motion of a satellite in geosynchronous orbit relative to Earth is typically a vertical figure eight. A person at a given point on Earth will observe a satellite in geosynchronous orbit at the same place at the same time of day every day.
A “geostationary orbit” is a specific case of geosynchronous orbit. In particular, a geostationary orbit is a circular, equatorial orbit in which a satellite remains stationary over a single point on the equator.
The geostationary Earth orbit (“GEO”) is a singularity, in that there is only one such natural orbit at an altitude of 35,786 km. If a parameter of the orbit, such as altitude or eccentricity, changes, the satellite will no longer remain stationary relative to the Earth's surface. Thus, a satellite in GEO is effectively locked into a given location relative to Earth. This allows the use of fixed ground antennas to continuously receive and send signals between the ground and the geostationary satellite. Direct-to-home broadcast satellites are one example of geostationary satellites.
Although geosynchronous orbits offer certain advantages, they also have shortcomings. For instance, because their altitude, latitude, and longitude are predictable, or even fixed, satellites in geosynchronous orbits are more vulnerable to hostile action (e.g., attack by hostile spacecraft or ground-based threats, such as lasers and anti-satellite missiles). In addition, because satellites in geosynchronous orbits cannot change altitude, latitude, or longitude without risking a loss of contact with fixed ground antennas, they cannot execute large longitude or latitude maneuvers or altitude changes to evade threats, whether those threats are passive (e.g., orbital debris) or deliberate (e.g., hostile action).